The trouble with cargo insurance on the same "pay 5%" approach as ship insurance is that it would either be useless in many cases or give an incredibly easy money copying exploit.
The big question is: if you lose your cargo before your ship is destroyed - cargo hatch malfunctions, hatchbreakers - or, indeed, if you lose your cargo without your ship being destroyed ... should insurance cover that?
If the answer is no, then you can still lose huge amounts of money in any case where you're attacked by a pirate who actually tries to do their job. (Griefers would specifically start carrying hatchbreakers ; pirates would be considered worse than outright murderers)
If the answer is yes then there's an utterly trivial cooperative exploit where player 1 buys expensive cargo with a cheap ship (Tritium, Gold, etc.), player 2 hatchbreaks the cargo and then destroys player 1, player 1 gets 95% of the value back while player 2 has 100% of the value, repeat for massive profit.
And if you lose some cargo to a pirate, but only get paid cargo insurance if you also lose your ship, that adds a really silly incentive to avoid trying to get out alive. But if you get paid regardless that opens up other exploits.
Secondarily, there's also a question about what value should be paid back to the player for insured cargo. If it's the value they acquired it for, then cargo insurance helps traders but does absolutely nothing for miners who are risking far more in terms of cost and time if destroyed. If it's the galactic average, then there's a whole pile of goods where buying them, launching, and boost-ramming the station would actually turn a significant profit.
Interestingly, the original game UI in the 1.0 release did have a "cargo insurance" line. It hung around for quite a while saying "none" and then disappeared, presumably because Frontier couldn't figure out a way to maake it work that was actually worthwhile.